1. Field of the Invention
The present invention relates to an optical disk and a method for manufacturing a disk substrate for an optical disk. In particular, the present invention relates to an optical disk and a method for manufacturing a disk substrate, for an optical disk in which a bonded portion exhibits a good appearance.
2. Description of the Related Art
As shown in FIGS. 6 and 7, a known bonded optical disk 100 such as a DVD (Digital Versatile Disk) is made by bonding a pair of disk substrates 110, 120 by an adhesive 101, with information recording areas 112 and 122 opposed to each other.
The information recording areas 112, 122 are produced by imprinting, using disk substrate molding stampers. The stamper is held on a movable mirror-surfaced plate by an inner peripheral stamper holder. On the surface of each information recording area 112, 122 of the disk substrates 110, 120, annular grooves 115, 125 are formed by projections of the inner peripheral stamper holder. A pair of disk substrates 110, 120 are formed in the same mold using interchangeable different stampers, or in different molds having different stampers of different pieces of information. In either case, as the same inner peripheral stamper holder is used the annular grooves 115, 125 formed on the pair of the substrates 110, 120 are identical in shape and size. Numeral 105 in the drawings represents a center opening of the optical disk 100.
As described in Kokai (Japanese Unexamined Patent Publication) No. 11-120618, when the disks are bonded with the annular grooves 115 and 125 being opposed and registered, a leakage liquid 102 occurs or an air bubble 103 is produced. The leakage liquid 102 shown in FIG. 6 takes place, for example, when in an adhesive spreading apparatus, a position where the adhesive is applied in a doughnut shape to the lower disk substrate 120 is located on the outside of the annular groove 125 in the radial direction and comparatively close thereto, or when a suction force from the rotation axis side is too large compared with a centrifugal force to spread the applied adhesive. The air bubble 103 shown in FIG. 7 is produced, for example, when a position where the adhesive is applied in doughnut shape to the lower disk substrate 120 is located on the outside of the annular groove 125 in the radial direction and comparatively far away therefrom or when a suction force from the rotation axis side is small, compared with the centrifugal force to spread the applied adhesive. The leakage liquid 102 and the air bubble 103 are present on transparent portions of the disks which are located on inner sides of the information recording areas 112, 122 in the radial direction and which are not subject to vapor deposition of a thin film or printing. Therefore, the leakage liquid and the bubble make the appearance of the disk bad and may reduce the adhesive force between the disc substrates 110 and 120.
A solving technology to prevent the leakage liquid 102 or the air bubble 103 mentioned above is described in Kokai (Japanese Unexamined Patent Publication) No. 11-120618. In the technology disclosed in Kokai No. 11-120618, a liquid stopping groove is provided on a disk substrate which constitutes an optical disk. The liquid stopping groove has a plurality of steps having different depths. The depth of the steps become deeper as the distance from the center of the disk is decreased, and the inside wall of each step is tapered. However, in the technology described in Kokai No. 11-120618, the manufacturing cost is high because it is necessary to provide plural steps in the liquid stopping groove, thus leading to difficulty in production of a mold.